In the past robotic manipulators have generally employed a six-degrees-of-freedom arm with an end effector, i.e. hand, capable of only simple grasping. Typically such end effectors used in the past have included two vise-like clamping fingers having together only one-degree-of-freedom of movement, due to being coupled together for movement purposes. Two problems that exist with such end effectors are that they are (1) unable to adapt to a wide range of object shapes; and (2) unable to make small displacements at the hand without moving the entire manipulating arm. This limits the response and fidelity of force control to that of the entire manipulator arm, even for very small motions. For example, often the most critical and necessarily accurate motions in a robot-controlled assembly task are of a very small magnitude in comparison to the range of motion of the entire arm. For example, once the manipulator has placed an object to be assembled in contact with another object, the motion necessary to complete the assembly may be less than one centimeter with an angular movement of less than 20 degrees. To make such motions it is necessary to very finely control the joints of the arm itself, which joints are typically designed to move through working volumes of one-half meter in radius or more.
One solution of this problem has been an attempt to use robot manipulators having actuators for the hands located on the forearm of the manipulator. Such actuators have been connected to the fingers by bare cables passing over a plurality of pulley sheaves in the manner of vintage dentist's drills. The wrists of such robot arms are complicated gimbaled affairs requiring more than one gimbal per cable. Consequently, the number of fingers and the degrees of freedom of each finger are severely limited by the complexity and crowding with the wrist mechanism. This limits the capability of such end effectors, i.e. hands, and makes them slow and cumbersome to manipulate and control, comparing quite poorly with actual human hand. Further, the weight and inertia of such hands require that they be specially designed together with the arm on which they are to be mounted and with painstaking attention having to be paid to the effects on their control parameters.
Another solution to the problem has been the attempted use of small motion-producing devices between the end effector and manipulator arm. The remote center compliance device discussed in Drake, "Using Compliance in Lieu of Sensory Feedback for Automatic Assembly," Charles Stark Draper Laboratory Report T-657, September 1977, is an example of a passive approach to such motion-producing devices between the end effector and the arm. A three-axis force-controlled assembler developed by Hill at SRI is an example of an active small motion device. However, both of these attempted solutions used end effectors suitable only for static grasping rather than combining the moving and grasping functions. This approach limits manipulative ability by (1) the lack of a stable adaptive grasp necessitating tool changes or limiting the class of manipulatable objects; and (2) placing the mass of the gripper and its actuator after the small motion device, thereby imposing a lower bandwidth on motion of the manipulator and end effectors at a given power. In addition, since these were capable only of grasping, the manipulative function was carried out for the most part by the manipulative arm itself.
Numerous designs have been presented for multifinger hands suited for grasping only. Some examples of these are shown in Skinner, "Designing a Multiple Prehension Manipulator," Mechanical Engineering, September 1975; Crossley & Umholtz, "Design for a Three Fingered Hand," Mechanism and Machine Theory, Vol 12, 1977; Childress, "Artificial Hand Mechanisms," Mechanisms Conference and International Symposium on Gearing and Transmissions, San Francisco, California, October 1972; Rovetta, "On Specific Problems of Design of Multi-Purpose Mechanical Hands in Industrial Robots," Proceedings 7 ISIR, Tokyo, 1977. Such designs have generally been aimed at approximating a subset of human grasping patterns observed to be useful in human functions.
A three-fingered design with a total of 11 degrees of freedom has been described in Okada, "Computer Control of Multi-Jointed Finger System," Sixth International Joint Conference on Artificial Intelligence, Tokyo, Japan, 1979. Okada uses a heuristic combination of position and force control fingers to grasp objects and impart some limited motion. While it is capable of some independent small motions, in an anthropomorphic manner, the Okada device requires a special arm design to accommodate it and a special control system. The Okada device does not address the problem of general motion of grasped objects.
The Okada apparatus has two cables per degree of freedom, i.e. joint for a total of 22 cables, all of which were passed through wrist gimbals. The arm is therefore severely limited and massive in size. In particular, the bare cables passing through the wrist joint cause a high degree of mechanical complexity and limited wrist motion. Moreki, "Synthesis and Control of Anthropomorphic Two-Handed Manipulator," Proceedings of the Tenth International Symposium on Industrial Robots, Milan, Italy, 1980 discloses that the number of control cables for each end effector, i.e., hand, can be significantly reduced. Moreki showed that for n degrees of freedom, n+1 cables can be used, and separate cable tensioning would not be needed.
One of the co-inventors of the present invention has pointed out, as co-author of a paper, Salisbury & Craig, "Articulated Hands: Force Control and Kinematic Issues," Proceedings of the 1981 Joint Automatic Control Conference, Charlottesville, Virginia, 1981, that contact by as few as seven frictionless points can immobilize a wide range of objects, i.e. grasp them securely against falling in any direction. However, the resulting implementation and design problems are formidable. In addition, an object held this way could not be rotated against resistance if it were a surface of revolution, e.g. a door knob or a cylindrical handle.
It is therefore quite desirable to have an end effector, i.e. hand, with a reasonable number of degrees of freedom and which has its actuators mounted, e.g. on the more massive forearm of the manipulating arm. It is also quite desirable to eliminate passage of the actuator's cables operatively through the mechanism of the wrist, thereby limiting the complexity of the wrist and avoiding multiple wrist gimbals. Such a hand would have the advantage of being transferrable from arm to arm if properly designed to be adapted to fit at the end of different arms, and would thereby be universal in its applicability. Such a hand would also improve present performance of robot manipulators, while reducing costs and enhancing the commercial applicability of robot manipulators.
The problems enumerated in the foregoing are not intended to be exhaustive, but rather are among many which tend to impair the effectiveness of previously known robot manipulating arms and end effectors. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that robot manipulator arms having end effectors appearing in the art have not been altogether satisfactory. Recognizing the need for an improved end effector for a robotmanipulating arm, it is, therefore, a general purpose of the present invention of providing a novel end effector which minimizes or reduces the problems of the type previously noted. A feature of the present invention employs using sheathed actuator cables which are operatively routed entirely outside the wrist gimbals enabling the hand to be used on a wide range of robot manipulator arms with only minor adaptations and to be universally mountable on a wide variety of manipulator arms. An additional feature of the present invention is to employ a plurality of, e.g. three fingers, having, e.g. three joints each. Still another feature, which may be employed with the present invention, is to employ frictional contact areas at the tips of each of the fingers to facilitate rotating all gripped objects, including those having surfaces of revolution, and reduce the necessary number of contact points for firmly grasping the object to be gripped. For example, three fingers, especially with friction contact, can easily grasp and rotate a sphere and a wide variety of other object shapes, because of the wide range of permitted grip positions. It is a further feature of the present invention to provide a robotic hand which can accomplish a wide range of rapid, small and precise motions on a wide range of objects sizes and shapes without the need of involving the remainder of the manipulator arm. The further feature of the present invention is to provide sheathed cables from the cable drive motors mounted remotely on the, e.g. forearm of the manipulator arm, which eliminates the need for routing pulleys, except in the joints in the end effector itself.
A further feature of the present invention is to obtain force and torque data by measuring the strain on the support structure, for example, those carrying the pulley sheaves over which the cables pass in, e.g. the palm of the hand.
Examples of the the more important features of the present invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which also form the subject of the appended claims. Other features and advantages of the present invention will become apparent with reference to the following detailed description of a preferred embodiment thereof, in connection with the accompanying drawings, wherein like reference numerals have been applied to like elements, in which: